1. Field of the Invention
The present invention generally relates to a signal transmission system and a signal transmission method. More particularly, the present invention relates to a radio frequency repeater in a signal transmission system used in a space in which plural radio systems exist concurrently.
2. Description of the Related Art
Since radio communication technologies have progressed in recent years, various radio communication systems have been proposed, and standardization and commercialization of the various radio communication systems are being carried out. Therefore, there are some cases where plural radio communication systems concurrently exist in the same region.
FIG. 1 is a schematic diagram showing a part of a space in which such plural radio communication systems exist. FIG. 1 shows plural mobile terminals 12-1, 2 and 3, plural antennas 14-1, 2 and 3, plural amplifiers 16-1, 2 and 3, a synthesizer/distributor 18, and a radio base station (node B) 20. The number of the antennas and he amplifiers can be arbitrarily determined. The mobile terminals 12-1˜3 are placed in an indoor closed space 10, and each of the mobile terminals 12-1˜3 performs radio communications using an antenna near the mobile station. Each of the antennas 14-1˜3 is provided at a predetermined position. For example, the antennas 14-1˜3 are placed at intervals of 20 meters. Alternatively, the antennas may be placed in different densities according to obstacles. Each of the amplifies 16-1˜3 amplifies a signal that passes the amplifier. The synthesizer/distributor 18 synthesizes UP LINK signals from the antennas 14-1˜3, and distributes DOWN LINK signals from the radio base station 20 to each antenna. The radio base station 20 is connected to a radio network controller (RNC) not shown in the figure.
For the sake of simplicity of description, it is assumed that the mobile terminal 12-1˜3 and the radio base station 20 adopt a Wideband-Code Division Multiple Access (W-CDMA) scheme such as the IMT2000 system, and that radio communications other than the W-CDMA scheme such as a Personal Handy Phone System (PHS), the Personal Communication Service (PCS) system, a Wireless Local Area Network (WLAN) and the like can be performed in the indoor closed space 10. In addition, it is assumed that the plural antennas 14-1˜3 are shared by the plural radio communications, and are used for both of transmitting and receiving. For the sake of simplicity, components such as a radio base station for PHS are not shown in the figure.
A DOWN LINK signal transmitted from the radio network controller (RNC) is received by the radio base station 20, sent to the synthesizer/distributor 18 via a transmission cable 19, and is distributed to each of the antennas 14-1˜3 so that the distributed signals are transmitted to the mobile stations 12-1˜3. The DOWN LINK signal is also called a downlink signal, outbound signal and the like. On the other hand, UP LINK signals from the mobile terminals 12-1˜3 are received by the antennas 14-1˜3, synthesized by the synthesizer/distributor 18, and are transmitted to the radio base station 20 via the transmission cable 19 so that the signals are transmitted to the radio network controller (RNC). The UP LINK signal is also called an uplink signal, an inbound signal and the like.
FIG.2 is a schematic diagram showing frequency spectrums for the UP LINK signal received by an antenna (the antenna 14-2, for example) in the indoor closed space 10. FIG. 2 shows three frequency bands corresponding to three radio communications systems existing together. As shown in the figure, a band of 1920˜1980 MHz is used by the W-CDMA scheme, a band of 1895˜1920 MHz is used by the PHS, and a band of 2400˜2497 MHz is used by the wireless LAN scheme. When plural mobile terminals operating under different radio communication systems are located nearly equidistant from the antenna, interference between the radio communication systems is small so that the mobile terminals can perform communications well under each communication system.
Japanese Laid-Open Patent Application No. 2002-198867 discloses a communication technology when plural radio communication systems exist concurrently.
However, plural mobile terminals operating under different radio communication systems are not necessarily always located nearly equidistant from the antenna. For example, as shown in FIG. 3, assuming that mobile terminals 32-1 and 2 (WiFi terminals conforming to the IEEE802.11b standard, for example) exist near the mobile terminal 12-2 that is performing radio communications using the antenna 14-2, and that the mobile terminal 32-2 is closer to the antenna 14-2 than the mobile terminal 12-2 is. In this case, as shown in FIG. 4, the signal for the wireless LAN becomes large in the antenna 14-2 so that a spurious component or an interference signal component that affects the communication signal for W-CDMA increases. In other words, a signal-to-noise ratio of the UP LINK signal transmitted from the mobile terminal 12-2 to the radio base station 20 deteriorates. The radio base station 20 or the upper radio network controller (RNS) instructs the mobile terminal 12-2 to transmit signals with a larger power to improve the signal quality of the mobile terminal 12-2. This instruction is transmitted by using a DOWN LINK signal. Then, the mobile terminal 12-2 performs radio communications with a larger transmission power.
In the W-CDMA scheme, since power control of the transmission power is properly performed for solving the near-far problem of the mobile terminal, there are few cases where a terminal of W-CDMA causes a large interference to another mobile terminal. However, such power control is not performed in radio communication systems other than the W-CDMA scheme. Therefore, there is a risk in that a signal from a radio communication system using a frequency band near that of the W-CDMA scheme may become a spurious signal for communications of the W-CDMA scheme.
By the way, the UP LINK signals from the antennas are synthesized by the synthesizer/distributor 18 shown in FIGS. 1 and 3, and, then, sent to the radio base station 20. Therefore, when a power of a signal transmitted via the antenna 14-2 increases, it is necessary to increase a power of each of signals from other antennas 14-1 and 3 in order to keep signal quality of the signals from the antennas 14-1 and 3. That is, when a power of the UP LINK signal from the antenna 14-2 is increased, noise level (noise floor) measured in the radio base station 20 increases, so that each mobile terminal is instructed to transmit signals with a larger power, or communication is prohibited (mobile terminals 12-1 and 3 shown in FIG. 3, for example). The reason is that the distance between the antenna and the mobile terminal in which communications can be performed decreases when it is requested that the antenna receives a signal with a larger power due to increase of the noise floor. From the view point of a communication capacity, since the number of connectable mobile terminals decreases, the communication capacity decreases. In addition, it can be predicted that the communication is likely to suffer interruptions. Further, since the mobile terminal transmits the signal with the larger power, consumed power increases, which is disadvantageous especially for a small-sized mobile terminal.
As to interference from other radio communication systems for a DOWN LINK signal, the interference can be effectively reduced by providing a band-pass filter in the antenna or by changing a power setting value in the network side or the like. However, as to the UP LINK signal, it is not easy to set such condition to take such measure in each mobile terminal distributed to each individual. Therefore, a technology for reducing or eliminating the above-problem for the UP LINK signal is required.
FIG. 5 shows antennas 16-1˜3 in the indoor closed space 10, and cells 52-1˜3 each indicating a region in which communication using a corresponding antenna can be performed in a normal state. The normal state is a state where there is no above-mentioned increase of the noise floor. Cells 54-1-˜3 smaller than the cells 52-1˜3 indicate cells when the noise floor increases.
FIG. 6 shows a simulation result on a power received by an antenna using an indoor closed space model. The simulation result is under an environment in which visibility is good in the indoor closed space. The vertical axis indicates power level (dBm/MHz) when 50Ω termination. The lateral axis indicates one-line distance (meter) between the antenna and the mobile terminal. More strictly, the one-line distance is obtained as the square root of the sum of squares of a horizontal distance and a vertical distance between the antenna and the mobile terminal. In the simulation, the antenna in the indoor closed space is provided at a ceiling at a height of 3 meters above the floor. The curves in the figure are obtained by plotting a distance by which equal to or more than 90% of received power in the vertical axis can be kept under various conditions or models.
The curve 61 is a graph in a case where a mobile terminal of the W-CDMA scheme outputs an AMR signal (voice conversation signal) with a transmission power of 21 dBm, and the signal is attenuated with a space attenuation index 2. The space attenuation index 2 corresponds to a free space model in which a signal is attenuated in inverse proportion to a square of the distance.
The curve 62 is a graph in a case where a margin to a short section center value variation and shielding of waves due to movement of a human are considered in addition to the conditions for the curve 61. The radius of the cell is evaluated based on this graph.
The curve 63 is a graph showing effects of a spurious signal received by the antenna when a WiFi apparatus performs radio communications with a transmission power of 10 dBm while producing the spurious signal of −35 dBm. The space attenuation index is 2.
The curve 64 is a graph showing effects of a spurious signal received by the antenna when a WiFi apparatus performs radio communications with a transmission power of 10 dBm while producing the spurious signal of −60 dBm. The space attenuation index is 2.
The curve 65 is a graph showing effects of a spurious signal received by the antenna when a PHS terminal performs radio communications while producing the spurious signal of −21 dBm. The space attenuation index is 2.
The curve 66 is a graph showing effects of a spurious signal (251 nW/MHz) of a PHS terminal conforming to an improved standard.
As shown in the figure, the curves 64-66 are below the curve 62 in the full section. Therefore, when the PHS terminal or the WiFi terminal does not exist, or when the PHS terminal or the WiFi terminal exits only far from the antenna (at least when each terminal exists in nearly the same distance from the antenna), a relatively large cell radius such as equal to or more than 20 meters can be realized. However, the peak (−70 dBm/MHz) of the curve 63 is equivalent to a level near 18 meters of the curve 62. Therefore, when the WiFi terminal exists near the antenna (about 3 meters from the antenna), a radio wave emitted from a place at a distance of 18 meters or more from the antenna are blocked. Therefore, according to the simulation result, it can be understood that the cell radius is shrunk to about 18 meters.
FIG. 7 assumes the same conditions as FIG. 6 except that the indoor closed space is in poor visibility. Curves 71 to 76 are graphs obtained by using the same conditions or models as those of the curves 61 to 66 in FIG. 6. In FIG. 7, it can be understood that the attenuation rate of the signal is larger than that in FIG. 6. In FIG. 7, the peak (−70 dBm/MHz) of the curve 73 is equivalent to a level near 12 meters of the curve 72. Therefore, when the WiFi terminal exists near the antenna (about 3 meters from the antenna), a signal emitted from a place at a distance of 12 meters or more from the antenna does not satisfy a required SIR. Therefore, according to the simulation result, it can be understood that the cell radius is shrunk to about 12 meters.